1. Field of the Invention
This invention relates to an image signal interpolating apparatus, and more particularly to an image signal interpolating apparatus preferably applied to improve the resolution of an image by interpolating picture elements which had been thinned out by subsampling.
2. Description of Related Art
Conventionally, as the method for band compression or information reduction when image signals are recorded or transmitted, methods of thinning out picture elements of an original image at a certain interval have been widely used. As an example of such method, multiple sub-nyquist sampling encoding has been known.
As an example of subsampling, offset subsampling has been well known. In the two dimensional offset subsampling of shown in FIG. 6, sampling interval (Tx, Ty) in the horizontal direction (x-direction) and vertical direction (y-direction) is set to be double a picture element interval (Hx, Hy) of each original signal, and subsampling (.times.) proceeds alternately. In the case of offset subsampling, sampling points (.largecircle.) which are positioned adjacent vertically are offset each other by a half sampling interval (TX/2). As the result, the space frequency component in horizontal and vertical directions of the transmission band of the image signal after offset subsampling is widened to the space frequency in slant direction as shown in FIG. 7, and consequently thin-off processing is possible without remarkable visual picture deterioration.
When the offset subsampled image signal is displayed on a monitor or printed out, as shown in FIG. 8, picture elements between each sampling point must be interpolated with adjacent picture elements. Such interpolation processing allows frequency component in the slant area shown in FIG. 7 to pass, and prevent the frequency component in area which contains loopback point A from passing and functions as a space filter, thus this interpolation processing is recognized as a back-end filter in the sampling theory.
Offset subsampling is very effective method when a front-end filter is correctly used, but when a front-end filter is not sufficient because of, for example, constraint due to hardware or a front-end filter is not sufficient because widening of the transmission band, the deterioration of image due to aliasing noise is a problem.
As a method to reduce the aliasing noise, adaptive interpolation has been proposed. In this method, when subsampled image signal is interpolated, closely correlated direction is detected around a picture element to be interpolated, and depending on the detected result a plurality of different interpolation means are selectively used for interpolation.
In the adaptive interpolation, the interpolation accuracy depends seriously on the accuracy of direction detection of closely correlated direction and performance of interpolation means. Therefore, when the performance of individual interpolation means is insufficient for proper interpolation, or closely correlated direction is erroneously judged, not only the original signal component decreases but also aliasing noise increases. These disadvantages are a serious problem of this adaptive interpolation.